P20B7: Reductant Metering Unit Heater Control Circuit Low

P20B7 means the metering unit heater circuit reads low. Volkswagen gives this a concrete meaning for the 2014 Touareg CNRB: the control unit checks heater conductance at the start of heating and flags P20B7 below 0.33 siemens, so the element shows more than about 3 ohms; further stages at 0.18 and 0.04 siemens follow. In VW's logic, low therefore refers to conductance, not to voltage, and points to a degraded element, a high-resistance joint or a partly broken wire rather than a short to ground. Other makers may define the low case through the output-stage voltage instead, since their drivers differ. The module receives too little heat, and thawing after a frosty night takes longer.

Symptoms

• Warning lamp at the start of a cold-weather heating phase
• DEF dosing delayed in winter

Most likely causes

• Heater element resistance too high (aged or partly damaged)
• Contact resistance at the heater terminals
• Heater wire partly broken (few strands left)

What to check, in order

Start with the simplest. You can tick off each step.

1. Check the AdBlue/DEF level and quality, and do not add additives or water
2. With the connector unplugged, check whether the signal wire is making contact with ground.
3. Look for white crystals around the injector and the tank
4. Check connectors and heaters with the scanner
5. Run the dosing test available on your diagnostic tool
6. Replace parts only when the test points to that component

Good to know

• Repair difficulty: hard
• It can trigger limp mode.
• It affects emissions.

What each character of P20B7 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 0: Subsystem depends on the code range
• B7: Specific fault in that system

Frequently asked questions

Can I keep driving with code P20B7?

Deal with it soon. If the instrument cluster shows a countdown, the car may not restart once it reaches zero.

What causes code P20B7?

The most common causes are: heater element resistance too high (aged or partly damaged); contact resistance at the heater terminals; heater wire partly broken (few strands left).

Related codes

• P20B5Reductant Metering Unit Heater Control Circuit/Open
• P20B6Reductant Metering Unit Heater Control Circuit Performance
• P20B8Reductant Metering Unit Heater Control Circuit High
• P20BBReductant Heater A Control Circuit Low

Sources

• static.nhtsa.gov: MC 10074008 2280
• law.cornell.edu: 13 CCR 1968.2

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.